The present invention relates generally to the powering of circuit cards and particularly to limiting inrush current to a circuit card.
Some systems require that the time the system is turned off while swapping circuit cards should be minimized. This can be accomplished by the removal and insertion of the circuit cards while power is still applied to the system. This is typically referred to in the art as xe2x80x9chot swappingxe2x80x9d or xe2x80x9chot pluggingxe2x80x9d.
Hot swapping can create a number of problems. For example, as an inserted circuit card is powered by mating the two connectors, the input voltage applied to the circuit card steps to the voltage of the mating power source. Since the circuit card typically has input filter circuits that include capacitors, the step-up in input voltage causes a large current to flow as a result of the input capacitors charging. This is typically referred to in the art as the inrush current. The inrush current may exceed the current rating of not only the power source but any fuses and circuit breakers as well. Also, a high inrush current may cause the connector pins to erode more quickly than normal.
Another problem with hot swapping is that the physical mating of two connectors can potentially cause contact chatter. Contact chatter is the physical bouncing of the contacts that occurs as the contacts on one connector come into contact with the contacts on a mating connector. The contact chatter causes voltage and current pulses to occur as the contacts make and break the electrical connection. These pulses could harm the circuitry of the circuit card.
Many different approaches exist to limit inrush current and contact chatter. These approaches are typically designed to limit the current to some predetermined magnitude that the circuitry can handle.
An active inrush limiter circuit is typically complex in that it uses transistors, operational amplifiers, comparators, and/or logic devices to generate time delays in controlling the application of power to a connected circuit card. Another conventional limiter circuit comprises a simpler approach by using a capacitor and resistor circuit connected to a bipolar junction transistor (BJT) or metal oxide semiconductor field effect transistor (MOSFET).
These approaches, however, introduce problems of their own. For example, the active inrush limiters require that the inrush limiter circuitry have a sufficient voltage to control the active components (e.g., transistors, operational amplifiers, comparators). A BJT circuit requires that the voltages exceed the base-to-emitter turn-on voltage of approximately 0.6V, plus bias overhead voltages. A MOSFET requires that the voltages exceed the gate-to-source turn-on voltage (threshold voltage) of approximately 2-4 Volts, plus bias overhead voltages. Other active components require that the input voltages exceed the minimum operating voltage for the device. This voltage can typically be in the range of 1.5V to 5V.
A common problem with the above approaches is that total inrush current control is not possible unless the control voltages of the circuits are maintained or reach a correct level to properly bias the transistors of the circuit. If the transistors typically used have no initial power applied, they are in the off state and cannot control the inrush current or the contact chatter. There is a resulting need in the art for limiting inrush current and contact chatter in a hot swappable electronic circuit.
Embodiments of the present invention encompass an inrush limiter circuit. The circuit comprises a first field effect transistor having a source terminal, a gate terminal, and a drain terminal. The source terminal is coupled to an input power source and the drain terminal is coupled to an output of the inrush limiter circuit.
A capacitor is coupled between the drain terminal and the gate terminal of the field effect transistor. A switching device is coupled between the gate and source terminals of the field effect transistor. When the switching device is turned on, it applies a voltage to the capacitor in order to charge the capacitor to the input power source level.